Abstract
Two experiments were conducted to determine whether the increased serum LH which occurs within 12 hr after a luteolytic dose of PGF 2α is dependent upon changes in progesterone or estradiol secretion. In the first experiment, exogenous progesterone abolished the increase in serum LH caused by a subcutaneous injection of 25 mg PGF 2α in diestrous heifers, but not in ovariectomized heifers. In the second experiment, progesterone pessaries were removed at 6 hr after a subcutaneous injection of 25 mg PGF 2α. LH remained at pre-PGF 2α values while the pessaries were in place, but began to increase within 1 hr after they were removed. Blood estradiol also remained at pre-PGF 2α values until the pessaries were removed, and began to increase at 2 hr after pessary removal. We conclude that the increase in serum LH within 12 hr after PGF 2α treatment in diestrous cattle is dependent upon withdrawal of progesterone; it is not due to increased serum estradiol.
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